This invention relates generally to electronic indicators and more particularly, to a self-compensating fuel system for measuring fluid level and additional properties of a fuel in which the indicator is submerged.
Liquid level indicators are widely used in a variety of applications, including, for example, vehicle fuel tanks. Such indicators generally provide information on the volume of a liquid, for example fuel, remaining in a container such as a fuel tank. Known fuel level indicators include mechanical indicators such as those commonly used in toilet tanks, and more modern, passive capacitive electronic indicators which typically operate by sensing the effect of the dielectric property of a fuel on capacitance when the indicator is partly submerged in the fuel. Electronic sensors submerged in the fuel generate signals to an electronic circuit, which, in turn, provides an indication of the fuel level. Generally, the electronic indicators have potentially greater accuracy for level indication than the older, mechanical type, and electronic indicators are less costly to manufacture.
However, though electronic indicators are more desirable to manufacturers and consumers alike, the art has struggled to provide electronic indicators which are both reliable and accurate. More recently a capacitive liquid level indicator as described in U.S. patent application Ser. No. 09/145,675, filed Sep. 2, 1998, which is herein incorporated by reference, has been developed and is thought to have improved accuracy and reliability relative to other known capacitive indicators. The capacitive electronic liquid level indicator includes a dual array of capacitive sensors on a dielectric substrate, and is believed to provide improved accuracy and reliability over other known capacitive liquid level indicators by reducing the effects of lead capacitance and trans-substrate parasitic electric fields.
However, the known capacitive liquid level indicators, including that described in U.S. patent application ser. No. 09/145,675 are still limited. Known capacitive liquid level indicator have sensor pads of capacitor plates arranged in horizontal orientation with respect to the level of liquid being measured. Such an arrangement limits the accuracy of fuel level detection. Further, known capacitive level sensors do not take into account multiple factors that affect the reliability of the level measurement. Specifically, variations in the dielectrics of known fuels, the effect of temperature on the dielectric of a fuel, physical tolerances of the materials used, and manufacturing process variations all affect the accuracy and reliability of such capacitive sensors. For example, varying trace widths, variations in the dielectric of the substrate material, moisture in the substrate material, are all variables which affect the readings of such sensors.
Still further, known capacitive sensors are limited to providing only an indicator of liquid level. Additional, useful information pertaining to the liquid is not made available. This is a particular problem with liquid fuel in a vehicle fuel tank. Additional information on the current conditions and properties of the fuel would be extremely useful for improving the safety and efficiency of vehicle operation. For example, known capacitive liquid level indicators do not provide information on the chemical composition of the liquid. With the increased availability and use of chemical and mixed fuels such as flex-fuel, a vehicle operator may know the overall fuel level in the tank but not the relative amounts of the chemical components of the fuel. In addition, to avoid or avert freezing, it is useful for a vehicle operator to know the water content of a fuel. To assure the proper grade of gasoline actually delivered into tank, vehicle operators need to obtain an octane reading of the gasoline. Further, internal tank pressure as a measure of fuel vapor pressure within a tank would be useful for safety reasons. In addition, known capacitive liquid level indicators do not provide any external signals to actuate or control other devices, such as a fuel pump or a burglar alarm, in response to changing properties or conditions of the liquid.
Accordingly it would be desirable to provide a self-compensating capacitive fuel system that compensates for variations in the dielectric of the fluid being measured, as well as manufacturing variations, thus providing improved accuracy and reliability over known capacitive level sensors. It would also be desirable to provide a fuel system that provides both fuel level information and additional information on properties of a liquid fuel being measured. It would also be desirable to provide a fuel system that can provide information on the chemical composition of the fuel, including the water content, octane level of a gasoline, and ethanol content of a flex-fuel. It would be further desirable to provide a fuel system that provides signals to actuate or control other devices in response to changes in the fuel. It would be still further desirable to provide a fuel system that provides information on the internal pressure of a tank containing the fuel.
In an exemplary embodiment, a self-compensating fuel system includes a dual array of multiple capacitive sensors on a dielectric substrate, the dual array of capacitive sensors forming multiple sensor pairs. At least one signal detection circuit is further included, each signal detection circuit coupled to a first sensor pair and a second sensor pair. Thus, the fuel system includes at least two sensor pairs. In an exemplary embodiment the system includes more than two sensor pairs and mutiple signal detection circuits, each circuit coupled to a first and a second sensor pair. The signal detection circuits each include a comparator configured to compare the capacitance of the first sensor pair with the capacitance of said second sensor pair. By internally comparing the capacitance of the first sensor pair with the capacitance of the second sensor pair, the fuel system compensates for variations in the dielectric of the fluid being measured, as well as manufacturing and materials variations.
For example, the fuel system compensates in its fluid level indication for variables such as temperature and fluid composition that affect the accuracy and reliability of capacitive sensor readings. In alternative embodiments, the fuel system, by virtue of its passive electronic operation, provides a platform for providing additional circuitry for controlling other devices on a vehicle. The additional circuitry may be stand-alone, or may use the capacitive information provided by the sensors.
Each sensor pair corresponds to a discrete fluid level output, and so the number of sensor pairs is typically about six or eight, but can be much higher depending on the desired number of level indicator levels. One embodiment includes, for example, sixteen sensor pairs corresponding to sixteen levels. For large vehicles with large fuel tanks, greater numbers of sensor pairs are especially suitable. In an exemplary embodiment, the first sensor pair are positioned in staggered relation relative to one another so that at least one other, or third sensor pair is positioned between said first sensor pair and said second sensor pair. In another embodiment, the first sensor pair and second sensor pair are positioned immediately adjacent to one another. However, other relative positions of the first sensor and second sensor pair are possible.
To drive output devices using the signals from the sensors and signal detection circuits, the fuel system further includes a signal conditioning circuit for receiving the inputs from the signal detection circuit. The signal conditioning circuit is configured to drive at least one output device, including for example a fuel gauge for indicating fuel level to a vehicle driver. In one embodiment the signal conditioning circuit is configured to drive multiple output devices, including for example a fuel pump speed control and a fuel composition analysis circuit. Alternatively, fuel pump speed control circuitry and fuel composition analysis circuitry is stand-alone on the dielectric substrate.
In one embodiment, the fuel system includes a microcontroller or microprocessor, and the fuel composition analysis circuit. The fuel composition analysis circuit includes for example an electronic drive circuit, a complex impedance detection circuit, an electrochemical potentiostat and a temperature detector coupled in parallel, each configured to receive an input from a designated sensor pair and to provide an output to the microcontroller. The microcontroller is configured to analyze the outputs to determine the chemical composition of a fuel in contact with the fluid level sensor.
More specifically, the analysis circuit is configured to sense the chemical composition of the fuel by combining complex impedance measurements with temperature information, or to sense a change in the dielectric of the fuel as the concentration of a particular component of the fuel varies. The circuit is thus configured to sense, for example, the alcohol, water or octane concentration in gasoline or the layering of one liquid over another such as, for example, oil over water. In one embodiment, for example, the fuel composition analysis circuit is configured for signaling at least an alcohol content, such as the ethanol content, of the fuel. In another embodiment, the fuel composition analysis circuit is configured for signaling the water content of the fuel. In yet another embodiment, the fuel composition analysis circuit is configured for signaling at the octane rating of the fuel. The internal comparison of capacitance between the first sensor pair and second sensor pair provides a varying reference that is calibrated to the dielectric of the liquid being measured and thus compensates for variation in materials and manufacturing process. Further, by virtue of its passive operation, the fuel system provides a platform suitable for adding a range of electronic functions especially useful with respect to using and analyzing fuel in a vehicle. Such functions include the analysis of fuel chemical composition, fuel pump speed control, and the like. In alternative embodiments, additional electronic components include, for example, a temperature sensor, a hydrophone for sensing motion of the liquid, and a pressure sensor for sensing tank pressure. Such additional components are mounted or embedded in the substrate. In addition, the dielectric substrate may be flexible so that the sensors and additional components form a flexible circuit that can conform to the shape of a fuel delivery module or a fuel tank.
The fuel system is thus a passive, capacitive level sensor that self-compensates for changes in the dielectric of the fuel being measured, as well as materials and manufacturing process variations. In addition, the fuel system provides a range of information on the properties and conditions of a liquid fuel in a tank, including fuel level, fuel composition and quality, internal tank pressure, fuel temperature, and fuel movement while the car is unoccupied. Specifically, the fuel system provides information on the chemical composition of the fuel, including the water content, octane level of a gasoline, and ethanol content of a flex-fuel. The fuel system further provides output signals to actuate or control other vehicle devices in response to changes in the fuel or the tank.